The present invention relates to a polymer composition based on a thermoplastic elastomer and an ethylene-vinyl acetate copolymer, and more particularly to a polymer composition having good heat resistance and shape memory characteristics. It further relates to a formed product made of such a polymer composition and a method of producing such a formed product with good heat resistance and shape memory characteristics.
An ethylene-propylene copolymer, an ethylene-propylene-diene copolymer, etc. are elastomers which are widely used as rubber materials. However, since these elastomers have relatively poor heat resistance, they are usually cross-linked after having been formed into desired shapes.
In this case, they are sometimes blended with cross-linkable polymer components such as an ethylene-vinyl acetate copolymer and then cross-linked by heating. However, since such cross-linkable polymers are usually mixed with the elastomers at high temperature, such as 200.degree. C. or more, this may cause a cross-linking reaction to initiate before the composition is formed into its desired shapes. This in turn makes it difficult to mold or form the resulting blends to desired shapes. In other words, such elastomer blends have poor moldability and formability. Accordingly, a polymer composition, having not only high heat resistance but also good moldability and formability, cannot be obtained.
For instance, Japanese Patent Publication No. 59-50172 discloses a resin composition comprising (a) a copolymer such as an ethylene-vinyl acetate copolymer, (b) an ethylene-.alpha.-olefin random copolymer and (c) a polymer such as crystalline polypropylene in proper proportions, which is cross-linked to have a boiling xylene-insoluble gel percentage and a melt index kept at certain levels. The cross-linking is conducted by irradiation of with an electron beam. However, since this composition is blended at as high a temperature as 240.degree. C., the component (a), such as an ethylene-vinyl acetate copolymer, cannot be added in a large amount. As a result, the resulting composition has only 0.1-60 weight % of a boiling xylene-insoluble gel percentage after cross-linking. Because of this low gel percentage, this composition becomes sticky when heated at a high temperature.
Further, Japanese Patent Publication No. 62-22772 discloses a high-orientation film made of (a) 93-30 weight % of at least one polyolefin selected from the group consisting of an ethylene-vinyl acetate copolymer, low-density polyethylene, high-density polyethylene, crystalline polypropylene and polybutene-1, and (b) 7-70 weight % of a thermoplastic elastomer made of an ethylene-.alpha.-olefin copolymer, which has a boiling xylene-insoluble gel percentage of 60 weight % or less, a melt index of 1.0 or less, a tensile rupture strength of 5 kg/mm.sup.2 or more and a temperature, needed to produce a 20-% shrinkage ratio, of 85.degree. C. or less. This film is produced by extruding a resin having the above composition through an annular die at 180 .degree. -280.degree. C., solidifying it rapidly, cross-linking it by irradiation with a high-energy beam, and then stretching it. However, since the resin is blended and extruded:.at high temperatures, its cross-linking inevitably initiates during the steps of blending and extrusion. In addition, to enable the stretching of the film, the level of cross-linking by irradiation of high-energy beam must necessarily be low, resulting in a low boiling xylene-insoluble gel percentage.
Besides the heat resistance, it has been found that some types of elastomer compositions have shape memory characteristics, and various proposals have been made with respect to this property.
For instance, Japanese Patent Laid-Open No. 59-53528 discloses a shape memory norbornene polymer having a glass transition temperature of 10.degree. C. or more and a number average molecular weight of 1,000,000 or more. This polymer is molded and then deformed at a temperature lower than the molding temperature, and then the deformation of the molded product is set by cooling at a temperature lower than the glass transition temperature. When it is heated at a temperature between the glass transition temperature and the molding temperature, the deformed product can be returned to the original shape.
Japanese Patent Laid-Open No. 60-28433 discloses a synthetic copolymer, such as a butadiene-methyl methacrylate styrene copolymer, having a glass transition temperature higher than room temperature. Its cross-linked product has excellent shape memory characteristics and good formability.
Further, Japanese Patent Laid-Open No. 62-86025 discloses that a vulcanized rubber of a polymer, or its blend, such as a polychloroprene rubber having a glass transition
temperature of -10.degree. C. or less, a melting point of 35 .degree.-90.degree. C. and a crystallization degree of 10-50% has good shape memory characteristics and good rubber elasticity at room temperature.
However, any of the above conventional shape memory polymer compositions are not satisfactory because of any one of the following problems:
(a) When a norbornene polymer is used, its molecular weight is very large, leading to poor moldability and formability.
(b) Because of high crystallization temperature, the original shape cannot be regained without applying high temperature.
(c) Special conditions are needed for molding and forming.